[libcalamares] Add a pre-script for PythonJobs

This allows injecting arbitrary Python code before
the script of a module is even run. For testing
purposes, that gives us a chance to modify existing
(internal) modules before the script (e.g. to test
subprocess calls).

src/libcalamares/PythonJob.cpp

@@ -19,6 +19,8 @@
 
 #include <QDir>
 
+static const char* s_preScript = nullptr;
+
 namespace bp = boost::python;
 
 BOOST_PYTHON_FUNCTION_OVERLOADS( mount_overloads, CalamaresPython::mount, 2, 4 );
@@ -242,6 +244,11 @@ PythonJob::exec()
         calamaresNamespace[ "globalstorage" ]
             = CalamaresPython::GlobalStoragePythonWrapper( JobQueue::instance()->globalStorage() );
 
+        if ( s_preScript )
+        {
+            bp::exec( s_preScript, scriptNamespace, scriptNamespace );
+        }
+
         cDebug() << "Job file" << scriptFI.absoluteFilePath();
         bp::object execResult
             = bp::exec_file( scriptFI.absoluteFilePath().toLocal8Bit().data(), scriptNamespace, scriptNamespace );
@@ -319,4 +326,11 @@ PythonJob::emitProgress( qreal progressValue )
     emit progress( progressValue );
 }
 
+void
+PythonJob::setInjectedPreScript( const char* preScript )
+{
+    s_preScript = preScript;
+    cDebug() << "Python pre-script set to" << Logger::Pointer( preScript );
+}
+
 }  // namespace Calamares

src/libcalamares/PythonJob.h

@@ -41,6 +41,19 @@ public:
     QString prettyStatusMessage() const override;
     JobResult exec() override;
 
+    /** @brief Sets the pre-run Python code for all PythonJobs
+     *
+     * A PythonJob runs the code from the scriptFile parameter to
+     * the constructor; the pre-run code is **also** run, before
+     * even the scriptFile code. Use this in testing mode
+     * to modify Python internals.
+     *
+     * No ownership of @p script is taken: pass in a pointer to
+     * a character literal or something that lives longer than the
+     * job. Pass in @c nullptr to switch off pre-run code.
+     */
+    static void setInjectedPreScript( const char* script );
+
 private:
     struct Private;